This invention relates to voltage variable attenuators and more particularly, to monolithic integrated circuit attenuators using field effect transistors as voltage controlled variable resistors.
In electronic signal processing, a variable attenuator is one of the most versatile and widely applied components. Monolithic integrated circuit (MIC) attenuators utilizing field effect transistors have many advantages over the conventional PIN diode attenuators. PIN diode attenuators have non-linear attenuation versus control voltage characteristics and are very sensitive to control voltages at high attenuation levels. In addition, a complicated linearizing circuit is required for linear operation.
Monolithic microwave integrated circuit (MMIC) variable attenuators have been proposed that can replace PIN diode attenuators. The prior art MMIC variable attenuators utilize FETs as voltage controlled resistors.
The use of a field effect transistor (FET) as a voltage controlled variable resistor is well known. There is a region in the characteristic curve of a FET for small values of the drain to source voltage V.sub.DS, where the drain current I.sub.D varies linearly with V.sub.DS. The V.sub.DS value must be smaller than the gate to source voltage V.sub.GS less the threshold or pinch off voltage V.sub.T, i.e. (V.sub.GS -V.sub.T). The linear relationship between voltage and current allows the FET to be used as a voltage controlled variable resistor. In the linear region, the FET acts as a resistor whose value can be controlled by the gate voltage The drain can be kept either positive or negative with respect to the source when using the FET in this mode, which gives the FET the bilateral property of an ordinary resistor The resistance R.sub.DS of the FET in the linear region is given by ##EQU1##
The relationship shows the dependence of the resistance R.sub.DS on V.sub.GS. FIG. 1 is a plot of equation 1 showing the relationship between R.sub.DS and V.sub.GS for a typical single FET designed for use as a voltage controlled resistor. As a negative voltage applied to the gate is increased, the resistance varies substantially linearly between 0.0 and -2.0 volts until pinch-off at approximately -4.0 volts.
The characteristics of individual FETs fabricated in monolithic form are proportional to the gate width and other manufacturing parameters While the characteristics may vary slightly for individual FETs, the relationship between the resistance and the gate voltage is always in accordance with equation 1. The curve as shown in FIG. 1 may be moved up and down or compressed and expanded, by manufacturing the FET with a gate width that will yield the desired channel resistance However, the resistance-voltage relationship will remain in accordance with equation 1. In the many applications for voltage controlled variable FET resistors, there is often a need for a variable resistor having a voltage versus resistance relationship different than that of equation 1.
An example in the prior art of a monolithic attenuator using FETs as a voltage variable resistors is described by Fisher et al., "A Linear GaAs MMIC Variable Attenuator", RF Design, October 1987. Fisher teaches the use of three single FETs having a gate width based on a trade off between insertion loss and parasitic capacitance In addition, complicated external analog control circuitry is required to apply the desired gate voltages to each FET. The control circuitry includes active components requiring D.C. power to drive the circuit.
Another prior art attenuator using a FET resistor is disclosed by Lizama, et al., "1-6 GH.sub.z GaAs MMIC Linear Attenuator With Integral Drivers", IEEE 1987 Microwave and Millimeter-Wave Monolithic Circuits Symposium. Lizama, et al. uses two FETs in parallel having different gate widths to provide improved linearity of attenuation. Lizama, et al. uses a complex integrated driver circuit consisting of active elements to provide the various gate voltages This driven circuit also requires D.C. power, substantially increasing the cost of the attenuator, and its sensitivity to temperature variations and process changes.